1. Field of the Invention:
This invention relates to an information signal reproducing apparatus having a tracking control system and more particularly to an apparatus reproducing an information signal from a tape-shaped recording medium on which pilot signals of four different kinds having different frequencies are cyclically recorded one after another along with the information signal.
2. Description of the Prior Art
The conventional information signal reproducing apparatuses of the above-stated kind include a video tape recorder (hereinafter referred to as VTR) arranged to perform a tracking control operation in a manner called the four-frequency method. In the present specification, this invention is described with the VTR of this kind taken up by way of example.
The tracking control method of the VTR is as follows: FIG. 1 of the accompanying drawing shows a recording pattern formed on a magnetic recording tape by the conventional VTR. The illustration includes the magnetic tape 1; a tape travel direction 2; the tracing direction 3 taken by a magnetic head; recording tracks 4; the magnetic head 5; the frequencies f1, f2, f3 and f4 of pilot signals recorded, one in each of the recording tracks 4; and track numbers An and Bn assigned to these recording tracks. The magnetizing direction of the recording tracks numbered An is arranged to differ from that of the recording tracks numbered Bn. The so-called azimuth overlap writing is accomplished by this arrangement.
When a reproducing head 5 is mainly tracing one of the recording tracks 4 (hereinafter referred to as the main track), the pilot signals are reproduced also from adjacent tracks on both sides of the main track. The levels of the reproduced pilot signals from the two adjacent tracks are equalized with the head arranged to adequately trace the main track.
FIG. 2 shows an example of the conventional circuit arranged on the above-stated concept. Referring to FIG. 2, a low-pass filter (hereinafter referred to as LPF) 6 is arranged to separate a pilot signal component from the reproduced signal. A reference signal generating circuit 7 is arranged to generate a reference signal in the same frequency as that of the pilot signal recorded in the main track. The circuit arrangement includes a multiplier 8; band-pass filters (BPF's) 9 and 10; detection circuits 11 and 12; a comparator 13; and an inversion circuit 14.
Assuming that a frequency difference between the pilot signals recorded in two adjacent tracks is fH (which corresponds to a horizontal scanning frequency) or 3fH as shown in FIG. 1, the BPF 9 is arranged to separate an fH component and the other BPF 10 to separate a 3fH component. In other words, each of the reproduced pilot signals from the two adjacent tracks is obtained in the form of a difference from the reference signal by which they are multiplied at the multiplier 8. The comparator 13 compares these reproduced pilot signals. Since the directions in which the fH and 3fH components are obtained for one track become converse for a next track, the output of the comparator is produced from a switch 16 either via an inverting amplifier 14 or without passing the amplifier 14 alternately for every recording track. The output of the switch 16 becomes a tracking error signal. Further, assuming that a one-field (1/60 sec. in the case of an NTSC signal) portion of a video signal is recorded in one track, the operation of the switch is controlled by a rectangular wave signal of 30 Hz (hereinafter referred to as the 30 PG signal) which is synchronized with the rotation of the head 5. The reference signal does not have to be the same as the pilot signal recorded in the main track. For example, the inverting amplifier 14 and the switch 16 may be omitted by arranging the frequency sequence of the reference signals as f1, f3, f4 and f2 while that of the pilot signals of the main tracks is f1, f2, f3 and f4.
In obtaining the tracking error signal by the VTR of the above-stated kind, the kind of the pilot signal recorded in the main track is not always distinguishable. Even if the VTR is of a type arranged to have the frequency of the reference signal coincide with that of the pilot signal of the main track, the reference signal is not always in agreement with the pilot signal under a transient condition. Therefore, it is only possible to generate the reference signals in a predetermined rotation. Then, in the event of a prolonged transient condition, a reproduced video signal would deteriorate during the transient period of the tracking control.